1. FIELD OF THE INVENTION
The invention relates to frequency modulated or "chirp" transmitters such as employed in FM radar systems.
2. DESCRIPTION OF THE PRIOR ART
In the prior art, frequency modulation radar both of the linear and non-linear types is well known. The use of frequency modulation in both CW and pulsed radar has been extensively described in the technical literature. In an early form, frequency modulation was applied to CW radars in order to provide a means of range determination. In such radars, the continuously varying frequency of the transmitter waves provided a reference against which the time function (frequency versus time) of an echo signal was shifted with respect to the transmitted signal. In linear FM, the greater this shift, the greater the range of a given target.
The frequency modulation concept in radar has also been used to effect pulse compression. In accordance with that concept, a relatively long pulse may be transmitted and frequency modulated to provide a relatively large amount of power on target. Echo signals are passed through a matched filter (a dispersive delay line, for example ) so that the received pulses can be effectively narrowed. This technique is described in the technical literature including the text "Radar Handbook" by Merrill Skolnik (McGraw Hill, 1970), chapter 20 in particular. Chapter 2 of the text "Modern Radar, Analysis Evaluation, and System Design" by Raymond S. Berkowitz also provides a background in the FM pulse compression art.
One of the key problems associated with the use of FM swept pulses (sometimes referred to as "chirp"), is the problem of preserving the integrity of the FM function (frequency ramp) from ramp generator to antenna. The problem is particularly acute in wide band systems, i.e. systems in which the transmitter is pulsed with a wide band chirp. Parasitic oscillations and other extraneous signal effects may be produced in the components between the chirp generator and the antenna. Also, power sources may vary, tuning may change with temperature or material surface characteristics and the basis linearity of power amplification components cannot be assumed to be invariant.
It will be immediately realized that any such variation will tend to deteriorate the match between transmission and reception characteristics which is vital to FM chirp integrity. Range information derived from such a system can be subject to inaccuracy and scan patterns in frequency scan systems will tend to exhibit angle errors.
Highly stable and ultra-linear frequency ramp generators have been developed for the radar arts so that it is possible in the current state of the art to produce a frequency ramp in which the rate of change df/dt (where F is frequency and T is time) is constant to a very high degree of accuracy.
U.S. Pat. No. 4,038,612 describes one particularly sucessful apparatus for generating a highly linear frequency ramp. That device is also adapted to the generation of a non-linear freqwuency ramp since it responds to a repetitive digital program which times the zero crossings of the individual waves in the frequency ramp.
The manner in which the present invention advances the art by providing for preservation of the integrity of a frequency ramp once generated, as it passes through a power amplifier and other transmitting components responsive thereto, will be understood as this description proceeds.